Article surveillance system

ABSTRACT

An article surveillance system for detecting the presence of articles in a detection zone (18) has at least one transmitter means (11, 13) and at least one receiver means (12, 15) for transmitting and receiving, respectively, electromagnetic radio-frequency signals in the detection zone or in proximity thereof. Each article is provided with a sensor or marker (20) operating as a transponder for transmitting an electromagnetic radio-frequency reply signal to the receiver means when receiving a signal from the transmitter means. Furthermore, the system has a coil arrangement (16) with driving means (17) for generating a low-frequent magnetic modulating field in the detection zone (18), and a controller (14) operatively connected to the transmitter means (11, 13), the receiver means (12, 15) and the coil arrangement (16, 17). Each sensor (20) is arranged to transmit a reply signal, the amplitude of which is modulated by the magnetic modulating field, and the receiver means is arranged to receive and to demodulate the amplitude-modulated reply signal. The controller (14) is arranged to supply modulating signals ( i  mod) to the coil arrangement (16, 17), to receive the demodulated signals ( i  demod) from the receiver means (12, 15) and to use the demodulated signals ( i  demod) when determining the position of the sensor (20) in relation to the detection zone (18).

TECHNICAL FIELD

The present invention relates to an article surveillance system fordetecting the presence of articles in a detection zone, comprising atleast one transmitter means and at least one receiver means fortransmitting and receiving, respectively, electromagneticradio-frequency signals within the detection zone or in a proximitythereof, each article being provided with a sensor operating as atransponder for transmitting an electromagnetic radio-frequency replysignal to said receiver means when receiving a signal from saidtransmitter means.

DESCRIPTION OF THE PRIOR ART

For many years now a substantial need for simple and still reliablesurveillance systems for monitoring objects or articles within a givenarea has been noticed in various business and industrial applications. Acommon example is the electronic shop antipilferage systems, which areavailable in many different kinds.

According to a common kind of electronic article surveillance systemseach article is provided with a small label or marker, which comprises athin metal strip with magnetic properties. At both sides of the shopexit arc-shaped magnetic field generating means are arranged forgenerating a magnetic field there between. When an article, which hasbeen provided with an antipilferage label according to the above, iscarried in between the arcs, the metallic strip is affected by themagnetic field and a detectable physical change occurs in the metalstrip. It is common to make use of the fact that an alternating magneticfield will periodically switch the magnetic moment of dipole in themetallic strip. Alternatively, the metallic strip may be forced intomechanical resonance, provided that the material and dimenions of thestrip are chosen accordingly. These physical changes are inductivelydetected by the arcs, wherein an attempted theft may be registered.Since the detection is made by inductive means, antipilferage systems ofthis kind suffer from a short detection range of a few meters only,which means that the antipilferage arcs must be arranged close to eachother and which in turn makes the shop exit narrow and "unfriendly" forthe customers.

In addition various antipilferage systems of a more advanced type arepreviously known. For instance, U.S. Pat. No. 5,030,940 discloses anelectronic article surveillance system. Electronic labels are used formarking and theft-protecting the desired articles. Such an electroniclabel is of a radio-frequency transponder type and comprises forinstance an antenna, a power source, such as a battery, and a non-linearcircuit, for instance some kind of semiconductor diode. Through itsantenna the transponder may receive a first electromagnetic signal of ahigh frequency, which has been transmitted by a transmitter in thesurveillance zone, as well as a second signal of a substantially lowerfrequency, by means of which an electrostatic field is generated in thesurveillance zone. By varying the electrostatic field, certainproperties of the non-linear circuit are influenced, the most importantof which being the electric reactance. These variations in reactance areamplified by the power source. The antenna is connected to thenon-linear circuit, and hence a reply signal may be transmitted, whichaccording to the above is composed by the two signals received. When amodified reply signal is detected according to the above by a receiverin the surveillance zone, the system may determine the presence of anarticle within the surveillance zone and provide a suitable alarm signalas a consequence thereof. A drawback of such a system is the relativelyhigh price per unit due to the complexity of the sensor, thus making thesystem less suitable for instance for mass surveillance of articles in ashop. Furthermore, the electrostatic field may quite easily be shielded(so as to avoid detection), for instance by placing the article and thesensor inside a metallic housing, such as a bag covered with a metallicfoil on its inside. In addition, a major portion of the electrostaticfield will be shielded, if the article and the sensor are placed betweenhuman body parts, for instance in the armpit.

Normally, for basic antipilferage applications as described above, it isonly desired to determine the presence of a transponder or sensor in asurveillance zone, but not its identity or exact position in the zone.Such determination, however, is of interest in a similar technicalfield, such as price labelling of articles. A method and a device forthis purpose are disclosed in WO93/14478. A label acting as a sensor ortransponder is provided with an antenna and at least one electricresonance circuit, comprising inductive as well as capacitive means; aso-called LC-circuit. The resonance circuit is excited toself-oscillation by means of electromagnetic energy transmitted by anexcitation means and received through the antenna of the sensor. Byproviding the label with an amorphous magnetic element and controllingthe permeability of this element by means of an external heterogeneousmagnetic bias field, also the resonance frequency of the resonancecircuit may be controlled, since the change in permeability for theelement will affect the inductive properties of the resonance circuit.Due to the factors described above the frequency of the reply signaltransmitted from the resonance circuit is dependent upon the magnitudeand the direction of the magnetic bias field at the current position ofthe sensor in question. As a consequence a simultaneous detection of aplurality of identical sensors present in the surveillance zone ispossible, thanks to the reply signals thereof being separated in thefrequency domain through their different magnetic bias levels.Alternatively, a calculation "backwards" in three dimensions of theposition for any given sensor is possible by means of the detectedfrequency value, provided that the heterogeneous magnetic bias field isknown. By arranging a plurality of labels and/or amorphous magneticelements in predetermined positions with respect to each other, a givencode space may be obtained, wherein the reply signal may for instancerepresent an article number assigned to the article.

To combine requirements for a long detection range with requirements fora high degree of detection reliability, i.e. a minimum risk of falsealarms as well as non-detections, has proven difficult for the knownarticle surveillance systems described above. For systems with arelatively long detection range, such as for instance the one describedabove with reference to U.S. Pat. No. 5,030,940, there is a substantialrisk of articles located outside the intended detection zone beingreached by interrogation signals from the system, wherein such articleswill reply to these signals and give rise to a false alarm situation.The disadvantage of such a situation in shops etc may easily beimagined; an honest customer inside the shop carrying a protectedarticle close to the intended detection zone--the shop exit--may fullyunintentionally become the subject of a triggered antipilferage alarm.On the other hand, such false alarms are less likely in systems of thetype described above, where the detection is made inductively betweenmetallic arcs, but instead there is a disadvantage of having to arrangethe relatively large and bulky detection arcs close to each other and ata ground plane level, which results in a narrow and unfriendly shopexit.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an articlesurveillance system with a detection range long enough for being capableof monitoring a detection zone with a width of several meters as well aswith a well-defined detection zone and a minimum risk of false alarms.

Another object of the invention is to provide sectorization of thedetection zone with the ability of a more precise determination of theparticular sector within the detection zone, in which a monitoredarticle is located.

Yet another object of the invention is to enable detection of monitoredarticles, which are being transferred into and out of the detectionzone, respectively, as well as between sectors within the detectionzone.

These objects are achieved by an article surveillance system withfeatures according to the characterizing portion of the appendedindependent patent claims. Preferred embodiments of the invention aredefined in the subsequent dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following,reference being made to the accompanying drawings, in which

FIG. 1 schematically illustrates a block diagram for an articlesurveillance system according to the present invention,

FIGS. 2 and 3 are diagrams illustrating a physical effect, which is usedaccording to an embodiment of the invention,

FIGS. 4 and 5 schematically illustrate the propagation of a magneticmodulation field according to the invention,

FIG. 6 is a block diagram illustrating an electric circuitry, which isused in one embodiment of the invention,

FIG. 7 is a block diagram schematically illustrating the operationaccording to an embodiment of the present invention,

FIG. 8 is a schematic topview of a coil arrangement according to anembodiment of the invention,

FIG. 9 is a schematic sideview of a coil arrangement according toanother embodiment of the invention,

FIG 10 is a schematic topview of the coil arrangement according to FIG.9, and

FIG. 11 is a schematic topview of a modification of the coil arrangementaccording to FIGS. 9 and 10.

DETAILED DISCLOSURE OF THE INVENTION

FIG. 1 discloses a schematic block diagram illustrating an articlesurveillance system according to the present invention. For exemplifyingreasons the article surveillance system will throughout the descriptionbelow be described as an antipilferage system to be used in a shop.However, it should be obvious to any person reading the detaileddescription below, that the article surveillance system according to theinvention may be used also for other purposes than pure antipilferageapplications.

The reference numeral 10 represents a part of any given shop, which islocated close to the shop exit. Before an exemplary customer steps intothe exit area 10 on his way out of the shop, he has already passed sometype of cash-desk arrangement, where he is supposed to pay for all thearticles picked during his tour around the shop. For the purpose ofchecking that each customer actually has duly paid for all his articles,at least some, or even all, articles available in the shop assortmentare provided with a respective transponder-type sensor 20, which will bedisclosed in more detail below. When an article is paid for, the sensor20 is removed or deactivated at the cash-desk by conventional means. Anysensor 20 not being removed or deactivated will be detected by thearticle surveillance system according to the invention in a waydescribed below. For clarifying reasons neither the article, on whichthe sensor 20 is mounted, nor the person, who is making an illegalattempt to carry the article out of the shop without paying for it, havebeen indicated in FIG. 1.

A transmitter antenna 11 and a receiver antenna 12 are arranged in theexit area 10. The transmitter antenna 11 is operatively connected to anoutput stage 13, which in turn is connected to a controller 14. Theoutput stage 13 comprises various conventional driving and amplifyingcircuits as well as means for generating a high-frequency electriccurrent i_(HF), which will flow in alternating directions through thetransmitter antenna 11 when supplied thereto, thereby generatinghigh-frequency electromagnetic signals with a frequency f_(HF) in awell-known fashion around the transmitter antenna, which are transportedin different directions away from the antenna by propagation of waves.As described in more detail below, these electromagnetic signals areused for exciting a transponder or sensor 20 present in the exit area10, said transponder or sensor transmitting an electromagnetic replysignal when receiving electromagnetic energy from the transmitterantenna 11, and said electromagnetic reply signal being received by thereceiver antenna 12. Preferably, the frequency f_(HF) are selectedwithin the radiofrequent interval between, for instance, 100 MHz and 2GHz.

The receiver antenna 12 is operatively connected to an input stage 15,comprising conventional amplifying and signal-processing means, such asband-pass filter and amplifier circuits. Furthermore, the input stage 15is connected to the controller 14 and is arranged to forward a signal,which has been received and processed as described below, to saidcontroller 14.

Hence, the transmitter antenna 11 as well as the receiver antenna 12 arearranged for conventional conversion between an electric high-frequencysignal and an electromagnetic one. The antennas are for instance formedas conventional end-fed or center-fed half-wave whip antennas, but othertypes of conventional antennas are equally possible. Also the moreadvanced antenna types may be used, which for instance are capable ofgenerating electromagnetic fields with rotating or plane polarity,thereby giving an opportunity to control problems of a practical nature,such as reflection, absorption or directional dependency for the sensorsin the exit area. Furthermore, more than one transmitter antenna 11 andreceiver antenna 12, respectively, may be used.

Additionally, the controller 14 is operatively connected to at least onealarm device (not disclosed in FIG. 1), by means of which a conventionalalarm signal may be generated visually and/or acoustically, whenever thepresence of a sensor 20 has been detected as described below. Thecontroller may be realized in hardware alone, but since the signalanalyses described below may be carried out in software, i.e. as one ormore than one computer program executed in the controller, some kind ofcomputer is preferably used. Thus, in the following the controller isassumed to be a conventional personal computer, which has been providedwith interfaces necessary for the communication with other units in thearticle surveillance system.

Furthermore, a magnetic field-generating means 16 is arranged in theexit area 10, said means preferably being formed as a coil arrangementjust below the ceiling level, or between ceiling and roof. Thisarrangement has the aesthetic advantage of making the entire coilarrangement 16 less noticable or even invisible to the shop customersand thus providing a higher degree of freedom when designing the shopexit aesthetically appealing. The coil arrangement in the articlesurveillance system according to the invention is thus horizontallyarranged in contrast to the conventional systems described above, wherethe detection arcs are vertically arranged.

According to a basic embodiment of the invention the magneticfield-generating means 16 consists of an electric conductor, such as acopper wire, which is wound in one turn or a plurality of turns around acoil frame. Preferably, the coil arrangement is essentially shaped as arectangle, which is large enough to expose the entire part of the exitarea 10, which is to be monitored for passing customers, to a magneticmodulation field described below. According to one embodiment of theinvention the coil arrangement may consist of a plurality of coilsarranged next to each other, optionally with a certain degree ofoverlapping with respect to each other, as described below. Thedimensions of the coil arrangement with respect to the length and widththereof are in the order of a few meters, thereby obtaining a minimum offield strength variations between different positions within thedetection zone, which is advantageous for reasons of health and whichleads to minimum differences in the signal strength received atdifferent positions in the detection zone. The volume 18 essentiallydirectly below the coil arrangement 16 forms the detection zone of thesystem, i.e. the zone which a customer will also pass through whenleaving the shop.

Thanks to the invention a large but still well-defined detection zone ispossible (several meters wide combined with a transition area betweenthe states "inside" and "outside", respectively, of a few decimetersonly), as described below. The shop exit area 10 must of course bedesigned in such a way that the risk is minimized of having a customerpassing out of the shop through another way, deliberately or not, thanbelow the coil arrangement 16 and through the detection zone 18.

The coil arrangement 16 is connected to the controller 14 through adriving stage 17. The driving stage 17 comprises means for generating alow-frequency modulating current i_(mod), which is fed through the coilfor generating a low-frequency magnetic modulation field H_(mod) aroundthe coil, the propagation of which covers the entire detection zone butalso some areas outside this zone, which is more or less inevitable. Themodulating current is given a known variation in amplitude as a functionof time according to the generic formula i_(mod) (t)=ƒ(t). In its mostbasic form the modulating current corresponds to a pure sine waveformaccording to

    i.sub.mod (t)=Asin(2πf.sub.mod t),

where A as usual represents the amplitude of the current and f_(mod)represents the frequency thereof, but other more complicated mathematicfunctions are also possible.

When an electric current i is fed through a straight electric conductor,a magnetic field is generated, the field strength H of which is linearlydependent upon this current according to

H=i/2πr, where r represents the distance to the conductor,

and hence the magnetic modulation field H_(mod) generated as describedabove will vary according to the modulating current i_(mod). Thefrequency for H_(mod) may for instance be selected in the interval100-1000 Hz.

In the article surveillance system according to the invention everysensor 20 has such characteristics, that the electromagneticradio-frequency reply signal from the sensor may be controlled ormodulated by the magnetic modulation field generated by the coilarrangement 16. How to obtain such controllable characteristics inpractice is thoroughly described in the Swedish patent application9600528-5 previously filed by the present applicant and entitled "Sensorfor remote detection of objects", which has not been published yet, andin the following the sensor is assumed to be of the kind disclosed insaid patent application, for exemplifying but not limiting purposes.Other types of sensors may be used, as long as they fulfil thefunctional demands described below.

According to the patent application above each sensor is provided with apreferably wire-shaped element of an amorphous material with magneticproperties. A suitable material for this wire-shaped element is forinstance one of the amorphous alloys Co₆₈.1 Fe₄.4 Si₁₂.5 B₁₅ or Co₇₀.5Fe₄.5 Si₁₅ B₁₀, both of which have a major fraction of cobalt. Theelement is arranged in a dielectric environment, for instance inside aglasstube filled with a liquid, and the element is acting as a receiverantenna for incoming electromagnetic signals as well as a transmitterantenna for transmitting electromagnetic reply signals. The materialproperties of the wire-shaped elements are such, that the permeabilitygr thereof depends on the magnetizing field strength along the maindirection of the element in a way, which now will be closer described.

FIG. 2 graphically illustrates how the permeability gr of a materialaccording to the above depends on the magnetizing field strength H ofthe magnetic modulation field. In the lower portion of the diagram thesine wave variations of the latter is indicated. This variation causesthe instantaneous value of μ_(r) to be transferred, so to speak, alongthe graph between points A and B due to the variations in the modulatingfield. However, since the graph is symmetric with respect to the y-axis(i.e., μ_(r) is a function of |H|), μ_(r) varies twice as fast as H. Byadding a constant offset-component H_(offset) to the modulation fieldh_(mod) according to FIG. 3, μ_(r) is caused to vary between the valuesA' and B' at the same frequency as the modulating field. Theoffset-component H_(offset) may be generated by adding a DC-component tothe modulating current i_(mod). Alternatively, H_(offset) may berepresented by the magnetic field from the earth, which is alwayspresent.

The controllable permeability described above is used for generating anamplitude-modulated reply signal from each sensor 20. The electricimpedance of the element depends on the permeability μ_(r) of theelement material through the so-called giant magnetoimpedance effect inthe wire-shaped element, and since the permeability in accordance withthe above varies together with the magnetic modulation field, also theamplitude of the reply signal from the sensor will vary with the samefrequency.

At each moment the modulating field has a certain vertical directioninside the detection zone 18 (i.e. straight below the coil arrangement16), but an opposite direction almost immediately outside the coilarrangement, and according to one embodiment of the invention this factis utilized to make it possible to determine, when detecting a sensor20, whether the sensor is located inside the detection zone 18 (thusmotivating the generation of an alarm), or outside the zone (where analarm should not be generated in order to avoid the risk of having afalse alarm). By investigating the graph in FIG. 3 one realizes why thisis possible, as described below.

The offset-component H_(offset) and the modulating field H_(mod) areassumed to have identical and essentially vertical orientation downwardsinside the detection zone 18. The resulting modulating magnetic field isrepresented by the graph in the lower rightmost portion of the drawing,which varies as a sine function around the value H_(offset). When thefield strength of the magnetic field increases from its average value toits maximum value, which has been indicated by an arrow in FIG. 3, thepermeability μ_(r) will decrease from its average value towards theminimum value B', which also has been indicated by an arrow, and willthen again start to increase and follow the variations in the magneticfield according to a sine wave, as described above. On the other hand,H_(offset) and H_(mod) have opposite directions outside the detectionzone 18, which may be viewed upon as the H_(offset) value now beingnegative (-H_(offset) in the leftmost portion of FIG. 3). It appearsthat when the leftmost sine graph--in correspondence with thedescription for the rightmost graph--increases towards its maximumvalue, μ_(r) will initially increase, i.e. the sine wave variations inpermeability starts in a different phase than for the rightmost graph.The sine wave variations in permeability (and hence also the variationsin the amplitude of the reply signal from the sensor) are consequentlyoffset in phase by 180° with respect to each other, depending on whetherthe sensor is located inside or outside the zone 18.

After having demodulated the received sensor signal in the input stage15 as described below, the relative phase (between the modulatingcurrent i_(mod) and a demodulated reply signal i_(demod) may be used asa detection criterion:

    φ=0°→the sensor is located inside the detection zone 18,

    φ=180°→the sensor is located outside the detection zone 18.

FIG. 4 schematically illustrates the direction of the field vector ofthe magnetic modulating field at a certain moment of time. Apparently,the field vector in a certain position inside the detection zone(straight below the coil arrangement 16, which is illustrated incross-section) is substantially directed along a vertical directiondownwards, while the field vector for any given position outside thedetection zone 18 is instead substantially directed upwards.

FIG. 5 illustrates the z-component, i.e. the vertical component, of thefield vectors in FIG. 4. A large main lobe, where the z-component isdirected downwards, covers essentially the entire detection zone 18,while sidelobes comprising z-components directed upwards are presentoutside the detection zone.

A possible way of realizing the functions of the article surveillancesystem described above will now be described with reference to the blockdiagram in FIG. 6. Then a more detailed analysis of the particularmethod of detection will follow with reference to FIG. 7.

A voltage-controlled oscillator (VCO) 30 is controlled through a DCvoltage V_(cont) for generating a high-frequency signal in a well-knownmanner. The frequency of the high-frequency signal is for practicalreasons divided down to half of the original frequency by means of adivider circuit 31, and then the signal is bandpass filtered around thecarrier frequency f_(HF) by means of a filter 32 to be supplied to thetransmitter antenna 11. A radio-frequency reply signal transmitted froma sensor 20 is received in the receiver antenna 12, filtered by means ofa bandpass filter 33 and supplied to an input of a mixer circuit 34. Themixer circuit 34 receives at a second input the output signal from thedivider circuit 33 and amplitude-demodulates the high-frequency signalreceived in the receiver antenna 12. The signal thus demodulated isfiltered by means of a filter circuit 35, before it is supplied to anamplifier 36 and a lowpass filter 37.

At the same time the controller 14 controls the coil arrangement 16through the driving stage 17 for generating a low-frequency magneticmodulating field, as described above. Here, the driving stage 17comprises a sine wave generator 17a as well as a power amplifier 17b,and the modulating current i_(mod) supplied by the generator 17a is alsosupplied to a first input of a synchronous demodulator circuit 38, whichat a second input thereof receives the receiver signal i_(demod), whichaccording to the above has been amplitude-demodulated as well asamplified and filtered. The synchronous demodulator circuit 38 is, asdescribed below, arranged to mix the two input signals and to lowpassfilter the resulting signal, wherein a DC component is obtained, thesign of which (positive or negative) may be used to determine whether agiven sensor is located inside the detection zone 18 or outside thezone, as described below.

Now, the method of detection will be closer described with reference toFIG. 7, wherein the components described above are given the samereference numerals as in previous parts. The following signals arepresent in the system according to FIG. 7:

S₁ =A₁ cos((ω₁ t) is the transmitted high-frequency signal (the carrierwave),

S₂ =A₂ cos(ω₂ t)·cos((ω₁ t+φ₁) is the amplitude-modulated reply signalfrom the sensor,

S₃ =A₁ 'cos(ω₁ t+φ₂)+A₂ 'cos(ω₂ t)·cos(ω₁ t+φ₁) is the received signal,which contains components originating from the transmittedhigh-frequency carrier signal as well as the modulated reply signal, and

S₄ =A₃ cos(ω₂ t) is the low-frequency signal for generating the magneticmodulating field.

According to the above the signals S₁ and S₃ are mixed in the inputstage 15, thereby obtaining the signal S according to:

    S=S.sub.1 ×S.sub.3 =A.sub.1 A.sub.1 'cos(ω.sub.1 t)cos(ω.sub.1 t+φ.sub.2)+A.sub.1 A.sub.2 'cos(ω.sub.1 t)cos(ω.sub.2 t)cos(ω.sub.1 t+φ.sub.1)==1/2A.sub.1 A.sub.1 '[cos(2ω.sub.1 t+φ.sub.2)+cos(φ.sub.2)]+1/2A.sub.1 A.sub.2 'cos(ω.sub.2 t)[cos(2ω.sub.1 t+φ.sub.1)+cos(φ.sub.1)]

After filtering ω₁ as well as its harmonics the following expression isobtained:

    S'=1/2A.sub.1 A.sub.1 'cos(φ.sub.2)+1/2A.sub.1 A.sub.2 'cos(ω.sub.2 t)cos(φ.sub.1)

In the equation above cos(φ₁) and cos (φ₂) are dependent upon the sensorposition in relation to the antennas. Obviously, the second term is afunction of ω₂, i.e. the (angular) frequency of the modulating current.The synchronous demodulation is now effected by mixing S₁ ' and S₄ :

    S"=S.sub.1 '×S.sub.4 =1/2A.sub.1 A.sub.1 'A.sub.3 cos(φ.sub.2)cos(ω.sub.2 t)+1/2A.sub.1 A.sub.2 'A.sub.3 cos(φ.sub.1)cos(ω.sub.2 t)cos(ω.sub.2 t)==1/2A.sub.1 A.sub.1 'A.sub.3 cos(φ.sub.2)cos(ω.sub.2 t)+1/2A.sub.1 A.sub.2 'A.sub.3 cos(φ.sub.1)cos(2ω.sub.2 t)+1/2A.sub.1 A.sub.2 'A.sub.3 cos(φ.sub.1),

which when filtered yields:

    S.sup.(3) =1/2A.sub.1 A.sub.2 'A.sub.3 cos(φ.sub.1)

The value of this DC component is continuously analyzed by thecontroller 14, which may obtain several conclusions from this value. Forinstance, if no sensors 20 at all are present in the exit area 10, thenA₂ '=0, wherein the value of S.sup.(3) =0 too. Consequently, as long asS.sup.(3) =0, there is no reason for alarm. On the other hand, wheneverS.sup.(3) reaches a predetermined threshold value ≠0, the presence of atleast one sensor is indicated. Thanks to the directional dependency ofthe modulating field described above, the factor A₃ will be positive ornegative, respectively, depending on whether the sensor is locatedinside the detection zone or outside said zone, hence making the sign ofS.sup.(3) an appropriate alarm condition. The detection securityinherent in this system may be increased by requiring the value ofS.sup.(3) to deviate from zero during a time period long enough and/orby a margin large enough. The security may be further improved throughthe enhanced embodiments of the system described below.

As an alternative to the synchronous demodulation described aboveanother appropriate type of signal processing may be used fordetermining the presence of the sensor within the detection zone.Conventional frequency analysis, such as FFT or DFT, seems particularlysuitable.

According to one embodiment of the invention the detection zone 18 maybe sectorized in different sectors in order to facilitate the individualdetection of a plurality of sensors, which all are simultaneouslypresent in different parts of the detection zone. Such a procedure ishighly desired, if the shop in question is provided with a wide exit of,for instance, 5-15 meters. Several customers may pass through such anexit simultaneously, and whenever any of these customers carries anarticle provided with a sensor, a system corresponding to the basicembodiment described above will detect the presence of the sensor andgive an alarm. Then a most unpleasant situation emerges for the rest ofthe customers in the detection zone, since which one of the customerswho has committed the attempted theft is not immediately obvious.

Such undesired situations may be avoided by partitioning the detectionzone in different sectors invisible to the human eye according to FIG.8. FIG. 8 schematically illustrates a wide detection zone 18,corresponding to a shop exit viewed from the above. The walkingdirection of the customers is to the "north", i.e. from the bottom tothe top in the drawing. Here, the magnetic field-generating means 16 isrepresented by a coil arrangement consisting of a plurality of coils16a, 16b, 6c, said coils being arranged side by side, thereby coveringthe entire detection zone 18. Similar to the case above each coil isoperatively connected to the controller 14, which controls the operationof the coil arrangement, so that the first coil 16a generates a magneticmodulating field at a first frequency f₁, the next coil 16b operates ata second frequency f₂, etc. Once a received reply signal has beendemodulated, the controller may determine the sector, in which thesensor in question is located, by means of the frequency contents of thedemodulated signal. While a sensor present in a certain sector will beinfluenced also by the magnetic modulating fields originating from thesurrounding sectors, the received and demodulated reply signal will bestronger at the "correct" frequency (for instance f₁, if the sensor islocated in the first sector), due to the field strength being dependentupon the distance. The determined sector may then be indicated asdesired, for instance by activating a lamp arranged in the ceiling abovethe sector in question. Furthermore, it is possible to detect therelatively unusual situation, where two customers are carrying arespective protected article at the same time through a respectivesector, since each sensor will reveal its existance through a respectivefrequency component in the received and demodulated signal.

Instead of constantly driving each coil arrangement at its ownfrequency, as described above, an alternating driving schedule ispossible as an alternative. In that case the controller 14 will controlthe different coil arrangements in such a way, that the first coilarrangement is active, i.e. generates a magnetic modulating field,during a first time interval Δt₁, while the remaining coil arrangementsare inactive. During a subsequent time interval Δt₂ the next coilarrangement will be active, and so on. The procedure is continuouslyrepeated, and the time intervals may essentially be chosen arbitrarilyshort, with the restriction, however, that a sufficient number ofmodulating cycles must lapse during each time period. Time intervals ofapproximately 10-100 ms have proven suitable for a modulating frequencyof about 500 Hz. The controller may then determine the correct sectorfor a reply signal received from a sensor 20 somewhere in the detectionzone 18 by registering the particular time period (Δt₁, Δt₂, . . . ),during which the signal was received.

According to another embodiment of the invention different sectors arearranged at a certain degree of overlapping in relation to each other,as shown for instance in FIG. 9. Such an arrangement has theconsiderable advantage of allowing a substantially narrower detectionzone (with respect to the walking direction for customers passing outthrough the shop) as described below, thereby reducing the risk of falsealarms. Furthermore, the arrangement provides the information receivedand available to the controller 14 with one additional dimension ofinformation. This new dimension is the position history of the currentsensor, i.e. the controller will now have the opportunity to detect thedirection of the transfer of a sensor between different sectors withinthe detection zone.

In FIG. 9 a detection zone 18a, 18b, corresponding to a portion of ashop exit, is schematically illustrated as a side view. Two coils 16a,16b, which are a part of the coil arrangement 16, are arranged at acertain height above a ground plane 21, such as the floor in the shop.As appears from FIG. 9, one coil 16a is arranged slightly above theother coil 16b with a partial overlapping d with respect to the firstcoil. Note that the situation in FIG. 9 is of a schematic nature only,and hence the relation in size between d and the coils 16a, 16b,respectively, should not be regarded relevant. The two coils arearranged, so that a customer passing out through the shop exit willfirst pass through the detection sector 18a belonging to the first coil16a, then through the overlapping zone 25 and finally through thedetection sector 18b belonging to the second coil 16b, i.e. in adirection from right to left in FIG. 9. Consequently, the coils 16a, 16bare here arranged behind each other (in relation to the travellingdirection of the customer) in the detection zone, in contrast to thesituation in FIG. 8, where the different coils are preferably arrangedbeside each other. Within the scope of the invention these differentalternatives may be combined, however, so that the detection zone may besectorized in its longitudinal direction as well as its transversaldirection, as is schematically illustrated in FIG. 10.

By alternately driving one of the coils, while the other is inactive, itis possible, as previously described, to detect the particular sector,in which the sensor 20 is located for the moment. Thanks to theoverlapping of the coils a considerably increased detection accuracy isavailable in accordance with the explanatory example below.

Assume that an article, which has been provided with a sensor 20, isbeing transferred out from the shop by a less scrupulous individual.First, the sensor 20 will reach the position to the right in FIG. 9,that is inside the detection sector 18a. During the short period oftime, when the coil 16a is active, the controller will detect thepresence of the sensor inside sector 18a, which is represented by thecharacter "+". Then the coil 16b is active, wherein the controller maydetermine that the sensor is located outside sector 18b, which isrepresented by the character "-". Since the procedure is continuouslyrepeated, the situation may be described as a sequence of state-pairsdetected by the controller 14:

    "+- +- +- +- +- +- +- . . . "

When the sensor reaches the overlapping zone 25, the presence of thesensor inside sector 18a will still be detected during the first half ofthe detection cycle, i.e. state "+⃡. However, the sensor is now alsopresent inside sector 18b, giving the state "+" also during the secondhalf of the detection cycle. Thus the following sequence is obtained:

    "++ ++ ++ ++ ++ ++ ++ . . . "

Finally, when the sensor has passed through the entire overlapping zone25 and is now located at the leftmost position in FIG. 9, the followingsequence is obtained:

    "-+ -+ -+ -+ -+ -+ -+ . . . "

Thus, by the coil arrangement and driving method described above it ispossible to detect the transfer of a sensor 20 in the detection zone 18.Specifically, a considerably high degree of detection security isobtainable, since the alarm conditions may be chosen in such a way, thatat least a few, or even all, of the above state sequences must have beendetected, before an alarm signal is generated. Alternatively, aconsiderably reduced detection zone is possible by only requiring thestate sequence "++ ++ ++ ++ . . . " before generating the alarm signal.In practice, the detection zone 18 has in such a case been reduced tothe overlapping zone 25 only, which may be made very narrow, forinstance a few decimeters. Still another advantage of having anoverlapping arrangement is the elimination of the potential problemposed by the factor cos(φ₁) in the expression described above for thesignal S.sup.(3). As previously described, φ₁ depends on the position ofthe sensor in relation to the antennas 11 and 12. Since cos(φ₁) may takeon positive as well as negative values, the basic embodiment describedabove may in some situations have problems when determining whether thesensor is present inside or outside the zone. However, this problem iseliminated by the differential operating method according to thearrangement described immediately above, i.e. the differences between(or the changes in) the various signal components are used as alarmconditions.

Instead of driving the coils alternatingly during short time intervals,it is equally possible to drive the coils simultaneously at differentfrequences (cf. the methods previously described), wherein theinfluences from the different coils on the reply signal are separated inthe frequency domain rather than the time domain as a consequence.

According to another embodiment the coils are arranged as overlappingpairs, similar to the description above. Now, however, both coils ineach coil pair are driven by modulating currents having the samefrequency, but instead the relation between the amplitudes of therespective modulating currents varies with time, whereby the magneticmodulating field contribution from the respective coil will vary withtime, too, for a given position in the detection zone. For instance, ata certain point of time the current amplitude in the first coil equals0,2I, while the current amplitude in the second coil equals 0,8I. Atanother point in time the conditions are 0,4I and 0,6I, respectively,etc. Consequently, the sum of the current amplitudes is constant, whilethe distribution between the two coils varies with time. According tothe above the magnetic field below the respective coil essentially has avertical direction, while the field in a proximate area just outside thecoil has a different and substantially horizontal direction. Hence, aslong as a sensor is located below both of the coils (i.e., inside theoverlapping zone), both magnetic field contributions have verticaldirections. Even if the magnitude of each contribution varies in time,the sum of the two is always constant in accordance with the above,which means that a reply signal demodulated according to the above has aconstant value.

However, when the sensor is moved outside the overlapping zone, so thatthe sensor is now located below one of the coils but outside the otherone, the two magnetic field contributions no longer have the samedirection. Hence, the field vector will now rotate back and forth, andthe sensor will no longer experience a constant resulting magnetic fieldvector. A demodulated reply signal with a varying amplitude is obtainedhereby. This may be described as a second, "superimposed" amplitudemodulation of the reply signal from the sensor, wherein this amplitudemodulation does not show, as long as the sensor is located within theoverlapping zone, but will appear, as soon as the sensor leaves theoverlapping zone. This fact is used when determining the sensor positionin relation to the overlapping zone. A field picture as described abovemay for instance be obtained by driving one of the coils in the coilpair at a certain phase shift in relation to the other.

Some practical problems, which the present inventor has been faced withduring the development procedure, will now be briefly discussed togetherwith suggested solutions to these problems, as an end of the descriptionof the invention and the various embodiments thereof.

A problem with a sectorized detection zone, where the different coilsare driven at different frequencies, is that the magnetic field from onecoil is induced to some extent in the adjacent coils, thereby causing aless well-defined sector definition. Furthermore, applicable rules formaximum permitted field strength in public premises must be considered.When using a plurality of frequencies the field strength at everyfrequency must be added together, and the total field strength thusobtained is not allowed to exceed a given maximum value, therebynaturally limiting the available field strength at each frequency, i.e.from each respective coil. Thus, for several reasons it is appropriateto minimize the number of frequencies in the system. One way of reducingthe number of required frequencies by 50% is to drive one coil in a pairof coils at a certain frequency, while the adjacent coil in the pair isdriven at the same frequency but with a phase shift of 90°. Since thedriving signals will then be orthogonal, the above-mentioned problem iseliminated. Furthermore, fewer frequencies have the advantage of a moresimple kind of driving electronics. By such driving of coil pairs with acertain difference in phase with respect to each other, a correspondingphase difference may be registered in the received reply signal, when asensor passes between the two coils, which may be used for determiningwhether the sensor is present in one sector or the other.

In particular situations a further problem of ambiguity may rise in thatthe received and demodulated signal will contain a signal component,which cannot be attributed to a certain sector but, so to speak, matchesmore than one sector. Such a situation will occur, if the sensor has anorientation, where it in a certain position outside a given sector isexposed to a modulating field vector, which would have caused exactlythe same reply signal, if the sensor would have been located inside anadjacent sector. The situation is illustrated in more detail in FIG. 10,disclosing a top view of a coil arrangement, comprising three adjacentlyarranged detection sectors. Each sector comprises a pair of coils 16a-b,16a'-b'and 16a"-b", respectively. The two coils in each pair of coilsare arranged at a certain overlapping in relation to each other, asdescribed above, wherein an overlapping zone 25, 25', 25", respectively,is formed in a way similar to the above. As illustrated, eachoverlapping zone extends slightly outside the respective pair of coils,and the ambiguity problems mentioned above may occur in these extensionareas. As described above, the magnetic modulation field is verticallydirected in the central portion of the respective overlapping zone. Inthe extending portions, however, the field has an essentially horizontaldirection.

Now, by imagining the experiment of keeping a sensor 20 in a verticaldirection in the central portion of e.g. the overlapping zone 25', onewill obtain the same reply signal, as if the sensor would have been keptin a horizontal direction in either of the extending portions of theoverlapping zone 25'. Hence, ambiguity problems occur within the dashedareas in FIG. 10. These problems may be avoided by orienting adjacentsectors/pair of coils at a certain rotation with respect to each otherin the horizontal plane, as described in FIG. 11, thereby avoiding aportion extending from a certain overlapping zone 25' to reach into thecentral portions of the adjacent overlapping zones 25 and 25",respectively.

The descriptions above for the preferred embodiments of the inventionare to be regarded as embodiment examples only. Other embodiments maydeviate from the ones described above within the scope of the invention,as defined in the appended patent claims.

In particular, it is to be understood that even if the coil arrangement16; 16a, 16b, 16c; 16a-b, 16a'-b', 16a"-b" is described above as beingarranged at a ceiling or roof level in the monitored premises, it isequally possible to arrange the coil arrangement at a ground levelinstead, the detection zone thereby being formed by an arbitrary volumeessentially straight above the coil arrangement, instead of below thearrangement, as previously. Furthermore, it is possible to combine thealternatives above, i.e. to arrange coils above as well as below thedetection zone. Such an arrangement would have the advantage ofexhibiting magnetic modulation field(s) of essentially equal strengththroughout the entire detection zone.

What is claimed is:
 1. An article surveillance system for detecting thepresence of articles in a detection zone, comprising at least onetransmitter means and at least one receiver means for transmitting andreceiving, respectively, electromagnetic radio-frequency signals in adetection zone, or in a proximity thereof, each article being providedwith a sensor operating as a transponder for transmitting anelectromagnetic radio-frequency reply signal to the receiver means whenreceiving a signal from the transmitter means,characterized bya coilarrangement with driving means for generating a low-frequency magneticmodulation field in the detection zone, and a controller (14)operatively connected to the transmitter means, the receiver means andthe coil arrangement, wherein each sensor is arranged to transmit areply signal, the amplitude of which is modulated by the magneticmodulation field, wherein the receiver means is arranged to receive andto demodulate the amplitude-modulated reply signal, and wherein thecontroller is arranged to supply modulating signals (i_(mod)) to thecoil arrangement, receive demodulated signals (i_(demod)) from thereceiver means and use the demodulated signals (i_(demod)) whendetermining the position of the sensor in relation to the detectionzone.
 2. An article surveillance system according to claim 1,characterized by the controller (14) being arranged to use informationon the phase between the modulating and the demodulated signals(i_(mod), i_(demod)) when determining the position of the sensor (20) inrelation to the detection zone (18).
 3. An article surveillance systemaccording to claim 1, characterized by the controller (14) beingoperatively connected to an alarm device for visual and/or acousticindication of the determined sensor position or any consequence thereof.4. An article surveillance system according to claim 1, characterized bythe coil arrangement (16) being horizontally arranged in parallel to aground plane (21).
 5. An article surveillance system according to claim1, characterized by the coil arrangement (16) being constituted by anessentially rectangular coil, comprising a coil frame and an electricconductor wrapped in one or more turns around the frame and beingarranged at a certain level above a ground plane, the detection zone(18) being defined by an essentially parallel-epipedic volume betweenthe coil and the underlying ground plane (21).
 6. An articlesurveillance system according to claim 1, characterized in that thecontroller (14) is arranged to determine whether the demodulated signals(i_(demod)) are in phase with the modulating signals (i_(mod)) at acertain moment of time, thereby indicating that the sensor (20) islocated inside the detection zone (18), and, if this is the case, togenerate an alarm signal by means of the alarm device.
 7. An articlesurveillance system according to claim 1, characterized in that the coilarrangement (16) comprises a plurality of adjacently arranged coils(16a, 16b, 16c), by means of which a respective detection sector (18a,18b, 18c) is formed between a respective coil (16a, 16b, 16c) and anunderlying ground plane (21).
 8. An article surveillance systemaccording to claim 7, characterized by the controller (14) and thedriving means (17) being arranged to supply modulating signals ofdifferent frequencies to the respective coils (16a, 16b, 16c) forgenerating modulating fields of different frequency in each respectivedetection sector (18a, 18b, 18c).
 9. An article surveillance systemaccording to claim 7, characterized by the controller (14) and thedriving means (17) being arranged to supply modulating signals insequence to a respective coil (16a, 16b, 16c) during a respective timeinterval.
 10. An article surveillance system according to claim 8,characterized by the controller (14) being arranged to separate from thedemodulated signals i_(demod)) a signal component for each respectivecoil and being arranged to determine from these signal components thesector, in which the sensor (20) is located.
 11. An article surveillancesystem according to claim 10, characterized in that at least two of thecoils (16a, 16b, 16c; 16a-b, 16a'-b', 16a"-b") are arranged at anoverlapping (d) with respect to each other, thereby together forming anoverlapping sector (25), which is defined by a volume between theoverlapping area (d) and an underlying ground plane, the controller (14)being arranged to continuously compare the signal components from saidat least two coils for determining the position of the sensor (20) inrelation to the overlapping sector and/or for detecting a direction oftransfer for the sensor.
 12. An article surveillance system according toclaim 11, characterized by the controller (14) being arranged to supplymodulating signals to said pairs of overlapping coils (16a, 16b, 16c;16a-b, 16a'-b', 16a"-b"), wherein the resulting magnetic field vector ofthe magnetic modulating fields generated by the respective coils in eachpair of coils has a constant direction in any position inside theoverlapping sector (25) but a varying direction outside the sector. 13.An article surveillance system according to claim 7, characterized inthat at least two of the coils (16a, 16b, 16c; 16a-b, 16a'-b', 16a"-b")are arranged at a certain rotation with respect to each other in thehorizontal plane.
 14. An article surveillance system according to claim1, characterized in that the detection zone (18) is constituted by atleast a part of a shop exit.
 15. An article surveillance systemaccording to claim 1, characterized by said sensor (20) comprising anelement of an amorphous magnetic material, the permeability of saidelement being dependent upon the magnetizing field strength of themagnetic modulating field along a main direction of the element.